The term "wide band signals" is defined here to mean pulse encoded TV signals which depending on the coding scheme will range from 1.5 million bits per second (mbps) to 216 mbps. An essential component of a switching network designed to switch wide band signals is a switching element which can switch the digital signals at the required rate, is capable of being fabricated into large arrays, and has low power dissipation. With certain types of pulse encoding schemes, such as pulse width modulation, the integrity of each digital pulse must be maintained. Distortion of the digital pulse is often referred to as "jitter". Jitter results in the pulse width being altered and normally results from the switching element having varying turn-on and turn-off thresholds due to crosstalk between switching elements or design deficiencies of the switching element.
A commonly used technique for designing circuits for switching digital signals at high speeds is to use emitter coupled logic (ECL). One commercially available high-speed ECL switching device that is capable of switching digitized video is the HXA230-101 which is manufactured by the Signetics Corporation. This device consists of 128 switching elements and is capable of connecting 8 output terminals to either of 16 input terminals. Since this device uses conventional ECL circuits, it dissipates approximately 2.4 watts. The reason for such a high power dissipation is that in conventional ECL circuits, the switching element draws full power independent of whether the element is in the enabled state or disabled state. Power dissipation is thus a severe problem when trying to implement a large video switch using a device such as the Signetics device. For example, a switching network capable of connecting a hundred sources to any of a hundred destinations would have a power dissipation of approximately 300 watts even though only a maximum of a 100 switching elements would be in the enabled state at any one time. The high power dissipation makes the array less economical because of the cost of the power supplies and power distribution. In addition, it also limits the maximum size of the switching network since the arrays cannot be packaged in close proximity to one another.
In view of the foregoing, there exists a need for a switching element which has the capability of switching pulse video signals, has low power dissipation, and has minimal jitter. Furthermore, the switching element should be capable of being integrated in large numbers onto a single integrated circuit for use in large switching networks.